Surface Equilibria of Biological and Organic Colloids

Abstract

The surface tension of a solution will change with the passage of time if some phenomenon brings about a change in the concentration of the solution at the surface. Du Noüy,1 among others,2 has studied the decrease in surface tension due to the adsorption and orientation of molecules of non-volatile surface active solutes in the surface layer. The fact that the surface tension of a solution of a volatile, surface active solute in water, may increase while the solution is allowed to remain undisturbed in a capillarimeter has been established.3 The theory was advanced that the increase in surface tension is due to the evaporation of the solute being more rapid than the diffusion of the solute molecules through the solution into the surface layer, while the solvent molecules are replenished about as rapidly as they evaporate, thus the surface layer becomes more nearly pure solvent with the elapse of time. That a change in the concentration of the surface layer, rather than the extremely small change in the gross concentration of the solution, is the cause of the change in surface tension, is indicated by the relatively large increase in surface tension which was observed during a short time interval. The fact that a slight agitation of the capillary meniscus after some increase in surface tension had taken place caused a return to near the original surface tension also leads to this conclusion. The quantitative work on which this theory is based was carried out with aqueous solutions of ethyl acetate. The fact that ethyl acetate has a higher vapor pressure at 20° than water seems to have been responsible for the idea advanced by some workers that the solute must be more volatile than the solvent in order to have the surface tension increase to that of the solvent due to evaporation from the surface layer. Accordingly, this investigation with aqueous solutions of «-butyl acetate, a material with a lower vapor pressure than water at 25°, has been carried out. Materials Water.-Carefully redistilled water was used in checking the capillarimeters, in preparing the solutions and in the final washings of all apparatus used in these measurements. «-Butyl Acetate.-Eastman's best grade of «-butyl acetate was repeatedly dried over phosphorus pentoxide, treated with sodium carbonate and distilled in an all-glass apparatus until further treatment caused no measurable change in the density or in the surface tension of the material; 450 g. of ester distilling over within a range of 0.15° (1) Du Noüy,